1. Field of the Invention
This invention relates to fuel ignition systems of the pilot ignition type and more particularly, to a control arrangement for use in such systems for providing an interlock on start-up under certain failure conditions.
2. Description of the Prior Art
In known fuel ignition systems of the pilot ignition type, a pilot valve is operated in response to the closure of thermostatically controlled contacts to supply fuel to a pilot outlet for ignition by a suitable igniter to establish a pilot flame. A pilot flame sensing circuit detects the pilot flame and effects the energization of a main valve which supplies fuel to a main burner apparatus for ignition by the pilot flame.
Typically, the operation of the main valve is controlled by a relay of the flame sensing circuit which has normally open contacts connected in the energizing path for the main valve and maintains the main valve deenergized until a pilot flame is established. When a pilot flame is established, the flame sensing circuit energizes the relay which then operates to close its contacts to connect the main valve to an energizing circuit to permit the main valve to operate. However, for a circuit failure which permits the relay of the flame sensing circuit to be energized at start up in the absence of a pilot flame, both the pilot valve and the main valve will be energized, permitting fuel to emanate from the pilot outlet and the main burner.
Various interlock arrangements have been proposed in the prior art, as exemplified by the U.S. Pat. Nos. 3,499,055 to J. C. Blackett, 3,644,074 to P. J. Cade and 3,709,783 to J. S. Warren, in which the energization of the fuel valves of the system is dependent upon the sequential operation of relays. In such systems, the energization of the pilot valve is effected in response to operation of a control relay which can be energized only is a flame sensing relay is deenergized. Once energized, the holding relay is maintained operated over a holding path provided by contacts of the relay. Thereafter, the energization of the main valve is effected in response to the operation of the flame sensing relay when a pilot flame is established, but only if the control relay is energized.
In such systems, the operation of the flame sensing relay is effected over an electric control circuit which is energized in response to the closing of thermostatically controlled contacts, and thus, for a failure of the control circuit which permits the flame sensing relay to be energized in the absence of a flame, the energization of the flame sensing relay may be delayed for a time following activation of the system. Accordingly, under certain conditions, the flame sensing relay may remain deenergized long enough to permit the control relay to operate, resulting in the unsafe condition referred to above with the pilot valve and main valve operated, and the igniter deenergized.